1. Field of the Invention
The invention relates to a multi-layer circuit board, more particularly to a multi-layer circuit board which can achieve impedance matching to result in reduced high speed signal reflection and reduced electromagnetic interference.
2. Description of the Related Art
A conventional multi-layer circuit board with eight wiring layers generally has a thickness of 1.6 mm or 1.2 mm. Reference is made to FIG. 1, which shows a multi-layer circuit board with a thickness of 1.2 mm. As shown, the circuit board includes: first, second, third, fourth, fifth, sixth and seventh insulating substrates (F1), (F2), (F3), (F4), (F5), (F6), (F7) disposed sequentially one above the other; a first signal wiring layer (S1) disposed on one side of the first insulating substrate (F1) opposite to the second insulating substrate (F2); a first ground wiring layer (GND1) disposed between the first and second insulating substrates (F1), (F2); a second signal wiring layer (S2) disposed between the second and third insulating substrates (F2), (F3); a second ground wiring layer (GND2) disposed between the third and fourth insulating substrates (F3), (F4); a power wiring layer (POWER) disposed between the fourth and fifth insulating substrates (F4), (F5); a third signal wiring layer (S3) disposed between the fifth and sixth insulating substrates (F5), (F6); a third ground wiring layer (GND3) disposed between the sixth and seventh insulating substrates (F6), (F7); and a fourth signal wiring layer (S4) disposed on one side of the seventh insulating substrate (F7) opposite to the sixth insulating substrate (F6). The first, second, third, fourth, fifth, sixth and seventh insulating substrates (F1), (F2), (F3), (F4), (F5), (F6), (F7), the first, second, third and fourth signal wiring layers (S1), (S2), (S3), (S4), the first, second and third ground wiring layers (GND1), (GND2), (GND3), and the power wiring layer (POWER) are press-bonded to each other to form the circuit board with a thickness of about 1.2 mm. The first and fourth signal wiring layers (S1), (S4) are adapted to be mounted with electronic components (not shown) thereon.
Each of the first and seventh insulating substrates (F1), (F7) has a thickness (H4) of about 2.5 mil. Each of the second, fourth and sixth insulating substrates (F2), (F4), (F6) has a thickness (H3), (H1) of about 8 mil. Each of the third and fifth insulating substrates (F3), (F5) has a thickness (H2) of about 5 mil. Each of the first, third, fifth and seventh insulating substrates (F1), (F3), (F5), (F7) is made from a polyester prepreg. Each of the second, fourth and sixth insulating substrates (F2), (F4), (F6) is made from a fibrous core material that contains paper or glass fibers. In this structure, the first signal wiring layer (S1) has a first resistance (Rs1) with respect to the first ground wiring layer (GND1). The second signal wiring layer (S2) has a second resistance (Rs2) with respect to the first and second ground wiring layers (GND1), (GND2). The third signal wiring layer (S3) has a third resistance (Rs3) with respect to the third ground wiring layer (GND3) and the power wiring layer (POWER). The fourth signal wiring layer (S4) has a fourth resistance (Rs4) with respect to the third ground wiring layer (GND3). The first and fourth resistance (Rs1), (Rs4) are about 44 ohms. The second and third resistances (Rs2), (Rs3) are about 55 ohms.
FIG. 2 shows a multi-layer circuit board with a thickness of 1.6 mm. The 1.6 mm circuit board differs from the aforementioned 1.2 mm circuit board in that the thickness (H4) of the first and seventh insulating substrates (F1), (F7) is about 9.5 mil. As a result, the first and fourth resistances (Rs1), (Rs4) are about 76.4 ohms, and the second and third resistances (Rs2), (Rs3) are about 51 ohms. Due to their construction, the conventional circuit boards shown in FIGS. 1 and 2 suffer from the following drawbacks:
1. Serious High Speed Signal Reflection
According to the standard theoretical values determined by Intel, the resistance between two adjacent wiring layers for a circuit board during high speed signal transmission is preferably within the range of 55±10% Ω, i.e., between 49.5 Ω and 60.5 Ω. However, the value of the first and fourth resistance (Rs1), (Rs4) of the aforementioned conventional 1.2 mm circuit board falls outside the preferred range recommended by Intel, and there is additionally a difference of about 11 ohms between the value of the first and fourth resistances (Rs1), (Rs4) and that of the second and third resistances (Rs2), (Rs3). Such a difference will result in an impedance mismatch. Thus, when a high speed signal is being transmitted through the conventional 1.2 mm circuit board and passes from the first or fourth wiring layer (S1) or (S4) to the second or third wiring layer (S2) or (S3) reflection of the signal will result, thereby adversely affecting signal transmission. Likewise, the value of the first and fourth resistances (Rs1), (Rs4) of the aforementioned conventional 1.6 mm circuit board falls outside the theoretical range, and there is a large difference of 25.4 ohms between the value of the first and fourth resistances (Rs1), (Rs4) and that of the second and third resistances (Rs2), (Rs3). The signal reflection problem is therefore very serious. The reflection index of high-speed signals for the 1.2 mm circuit board can be calculated as follows:   ρ  =                    Zl        -        Zo                    Zl        +        Zo              =                            Rs1          -          Rs2                          Rs1          +          Rs2                    =      0.111      The reflection index for the 1.6 mm circuit board can be calculated in a similar manner to result in 0.199.
(2)Weakened magnetic flux counteraction: As reflection of high speed signals will generate standing waves, which will increase electromagnetic radiation of the high speed signals, the magnetic flux counteraction of the circuit board is weakened, thereby resulting in excessively high electromagnetic interference.